1. Field of the Invention
The present invention relates generally to document reproduction apparatus and more particularly to an improvement for such apparatus including means for generating nonlinear clock signals used to compensate for signal distortion resulting from the use of rotary scanning devices.
2. Description of the Prior Art
One of the more serious problems which confronts the designers of high speed document reproduction systems is the difficulty of obtaining uniform quality data over the length of a scan line. This difficulty often results from variations in the distance between the line scanning device and the respective data containing segments of the scan line such as is the case where rotating mirrors or other rotating light focusing or transmitting means are used as a scanning device for collecting light eminating from a line extending across a flat document surface.
More particularly, when using such devices, signal distortions are caused by the nonlinear relationship between the rotation of the shaft supporting the light focusing or collecting device and the length of scan line per increment of shaft rotation. In other words, where the shaft is positioned over the center of the document and is rotated at a constant speed, the shaft angle required to cause the light detecting device to scan a short segment of the scan line near the document edges is substantially less than that required to scan a like segment of the line at the middle of the document. It will thus be appreciated that some type of nonuniform clocking mechanism is required to enable the data to be sampled at nonuniform rates depending upon where along the scan line the data is being taken from.
Prior art attempts to solve this problem have included the provision of nonuniformly spaced timing marks on the shaft perimeter and the use of nonlinearly coded wheels synchronized with the shaft position coupled with mark/detecting devices for generating the requisite nonlinear clock pulses. However, the use of such mechanical solutions requires highly accurate alignment of the various mechanical components and in many cases imposes unreasonable manufacturing tolerance requirements. The problem is even further complicated where multiple data scanning devices are affixed to the same shaft in order to improve the duty cycle of the scanner, since not only must the various scanning devices be accurately positioned relative to the surface to be scanned, but they must also be accurately positioned relative to each other.